Hydrogen-selective metal membranes, membrane modules, purification assemblies and methods of forming the same

ABSTRACT

Membrane modules that contain one or more hydrogen-selective membranes, methods for preparing the same, and hydrogen purification systems, fuel processors and devices containing the same. In some embodiments, the membrane modules include one or more hydrogen-selective membranes supported on a support or screen structure, of which a variety of embodiments are disclosed. In some embodiments, the membrane or membranes are adhesively mounted on the screen structure during assembly. In some embodiments, the screen structure includes a plurality of screen members adhesively mounted together during assembly. In some embodiments, the screen structure includes a coating. The present invention is also directed to methods for reducing the thickness of hydrogen-selective membranes.

RELATED APPLICATIONS

The present application is a continuation application claiming priorityto U.S. patent application Ser. No. 10/196,329, which was filed on Jul.15, 2002, issued on Jul. 22, 2003 as U.S. Pat. No. 6,596,057, and iscontinuation-in-part application claiming priority to U.S. patentapplication Ser. No. 09/723,724, which was filed on Nov. 27, 2000,issued on Jul. 16, 2002 as U.S. Pat. No. 6,419,728, and is acontinuation-in-part of U.S. Pat. No. 6,152,995, which was filed on Mar.22, 1999 as Ser. No. 09/274,154. U.S. Pat. No. 6,596,057 also claimspriority to U.S. patent application Ser. No. 09/618,866, which was filedon Jul. 19, 2000, and is also a continuation-in-part of U.S. Pat. No.6,152,995. U.S. Pat. No. 6,596,067 also claims priority to U.S. patentapplication Ser. No. 10/003,164, which was filed on Nov. 14, 2001,issued on Oct. 1, 2002 as U.S. Pat. No. 6,458,189, and is a continuationof U.S. Pat. No. 6,319,306, which was filed on Mar. 19, 2001 as U.S.patent application Ser. No. 09/812,499 and which claims priority to U.S.Provisional Patent Application Serial No. 60/191,891, which was filed onMar. 23, 2000. The complete disclosures of the above-identified patentapplications are hereby incorporated by reference for all purposes.

FIELD OF THE INVENTION

The invention relates generally to hydrogen-selective membranes anddevices that form and/or purify hydrogen gas, and more particularly tomethods for forming hydrogen-selective membranes, hydrogen-selectivemembrane modules, hydrogen purifiers and fuel processors.

BACKGROUND OF THE INVENTION

Purified hydrogen is used in the manufacture of many products includingmetals, edible fats and oils, and semiconductors and microelectronics.Purified hydrogen is also an important fuel source for many energyconversion devices, such as fuel-cell systems, and especiallyproton-exchange-membrane fuel-cell (PEMFC) systems.

Hydrogen gas streams may be produced by fuel processors that producehydrogen gas by chemically reacting one or more feed streams. These fuelprocessors often require that the initial hydrogen stream be purifiedbefore the stream is suitable for use in a particular application, suchas a feed stream to a fuel cell.

An example of a suitable fuel processor is a steam reformer, whichproduces hydrogen gas by reacting a hydrocarbon or alcohol with water.Other examples of suitable fuel processors produce hydrogen gas byautothermal reforming, partial oxidation of a hydrocarbon or alcoholvapor, by a combination of partial oxidation and steam reforming ahydrocarbon or an alcohol vapor, by pyrolysis of a hydrocarbon oralcohol vapor, and by electrolysis of water. Examples of suitable fuelprocessors and fuel cell systems incorporating the same are disclosed inU.S. Pat. Nos. 5,861,137, 5,997,594 and 6,376,113, the disclosures ofwhich are hereby incorporated by reference.

Hydrogen-selective membranes formed from hydrogen-permeable metals, mostnotably palladium and alloys of palladium, are known. In particular,planar palladium-alloy membranes have been disclosed for purifyinghydrogen gas streams, such as hydrogen gas streams produced by steamreformers, autothermal reformers, partial oxidation reactors, pyrolysisreactors and other fuel processors, including fuel processors configuredto supply purified hydrogen to fuel cells or other processes requiringhigh-purity hydrogen.

To be economical, palladium and palladium-alloy membranes must be thin.For example, planar membranes are typically approximately 0.001 inchesthick. However, forming an extremely thin membrane tends to become moreexpensive from a manufacturing standpoint as the thickness of themembrane is reduced. Furthermore, extremely thin membranes are subjectto wrinkling during assembly into a membrane module containing one ormore hydrogen-selective membranes. A membrane that has one or morewrinkles is subject to premature failure due to stress fractures formingat the wrinkle. When such a failure occurs, impurities that otherwisewould be unable to pass through the membrane can now pass through themembrane, thereby reducing the purity of the product hydrogen stream andpotentially damaging the fuel cell stack or other hydrogen-consumingdevice with which the purified stream is used.

SUMMARY OF THE INVENTION

The present invention is directed to membrane modules that contain oneor more hydrogen-selective membranes, methods for preparing the same,and hydrogen purification systems, fuel processors and devicescontaining the same. In some embodiments, the membrane modules includeone or more hydrogen-selective membranes supported on a support orscreen structure, of which a variety of embodiments are disclosed. Insome embodiments, the membrane or membranes are adhesively mounted onthe screen structure during assembly. In some embodiments, the screenstructure includes a plurality of screen members adhesively mountedtogether during assembly. In some embodiments, the screen structureincludes a coating. The present invention is also directed to methodsfor reducing the thickness of hydrogen-selective membranes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a fuel cell system containing a fuelprocessor with a membrane module according to the present invention.

FIG. 2 is a schematic diagram of another embodiment of the fuel cellsystem of FIG. 1.

FIG. 3 is a schematic diagram of a fuel processor suitable for use inthe fuel cell systems of FIGS. 1 and 2 and including a membrane moduleaccording to the present invention.

FIG. 4 is a schematic diagram of another embodiment of the fuelprocessor of FIG. 3.

FIG. 5 is a schematic diagram of a hydrogen purifier containing amembrane module according to the present invention.

FIG. 6 is a fragmentary side elevation view of a membrane envelopeconstructed according to the present invention and including a screenstructure.

FIG. 7 is an exploded isometric view of another embodiment of a membraneenvelope constructed according to the present invention and including ascreen structure having several layers.

FIG. 8 is a cross-sectional view of the membrane envelope of FIG. 7.

FIG. 9 is a fragmentary isometric view of an expanded metal screenmember suitable for use in the screen structure of FIG. 7.

FIG. 10 is an exploded isometric view of another membrane envelopeaccording to the present invention.

FIG. 11 is an exploded isometric view of another membrane envelopeconstructed according to the present invention.

FIG. 12 is an exploded isometric view of another membrane envelopeconstructed according to the present invention.

FIG. 13 is an exploded isometric view of another membrane moduleconstructed according to the present invention.

FIG. 14 is a cross-sectional view of a fuel processor that includes amembrane module constructed according to the present invention.

FIG. 15 is a cross-sectional view of another fuel processor thatincludes a membrane module constructed according to the presentinvention.

FIG. 16 is an isometric view of an unetched hydrogen-permeable metalmembrane.

FIG. 17 is a cross-sectional detail of the membrane of FIG. 16 with anattached frame.

FIG. 18 is an isometric view of the membrane of FIG. 16 after beingetched according to a method of the present invention.

FIG. 19 is a cross-sectional detail of the membrane of FIG. 18.

FIG. 20 is an isometric view of the membrane of FIG. 16 with anabsorbent medium placed over an application region of one of themembrane's surfaces.

FIG. 21 is a cross-sectional detail of the membrane of FIG. 20.

FIG. 22 is the detail of FIG. 19 with a hole indicated generally at 60.

FIG. 23 is the detail of FIG. 22 with the hole repaired.

DETAILED DESCRIPTION AND BEST MODE OF THE INVENTION

A fuel cell system according to the present invention is shown in FIG. 1and generally indicated at 10. System 10 includes at least one fuelprocessor 12 and at least one fuel cell stack 22. Fuel processor 12 isadapted to produce a product hydrogen stream 14 containing hydrogen gasfrom a feed stream 16 containing a feedstock. The fuel cell stack isadapted to produce an electric current from the portion of producthydrogen stream 14 delivered thereto. In the illustrated embodiment, asingle fuel processor 12 and a single fuel cell stack 22 are shown anddescribed, however, it should be understood that more than one of eitheror both of these components may be used. It should also be understoodthat these components have been schematically illustrated and that thefuel cell system may include additional components that are notspecifically illustrated in the figures, such as feed pumps, airdelivery systems, heat exchangers, heating assemblies and the like.

Fuel processor 12 produces hydrogen gas through any suitable mechanism.Examples of suitable mechanisms include steam reforming and autothermalreforming, in which reforming catalysts are used to produce hydrogen gasfrom a feed stream containing a carbon-containing feedstock and water.Other suitable mechanisms for producing hydrogen gas include pyrrolysisand catalytic partial oxidation of a carbon-containing feedstock, inwhich case the feed stream does not contain water. Still anothersuitable mechanism for producing hydrogen gas is electrolysis, in whichcase the feedstock is water. For purposes of illustration, the followingdiscussion will describe fuel processor 12 as a steam reformer adaptedto receive a feed stream 16 containing a carbon-containing feedstock 18and water 20. However, it is within the scope of the invention that thefuel processor 12 may take other forms, as discussed above.

Examples of suitable carbon-containing feedstocks include at least onehydrocarbon or alcohol. Examples of suitable hydrocarbons includemethane, propane, natural gas, diesel, kerosene, gasoline and the like.Examples of suitable alcohols include methanol, ethanol, and polyols,such as ethylene glycol and propylene glycol.

Feed stream 16 may be delivered to fuel processor 12 via any suitablemechanism. Although only a single feed stream 16 is shown in FIG. 1, itshould be understood that more than one stream 16 may be used and thatthese streams may contain the same or different components. Whencarbon-containing feedstock 18 is miscible with water, the feedstock istypically delivered with the water component of feed stream 16, such asshown in FIG. 1. When the carbon-containing feedstock is immiscible oronly slightly miscible with water, these components are typicallydelivered to fuel processor 12 in separate streams, such as shown inFIG. 2.

In FIGS. 1 and 2, feed stream 16 is shown being delivered to fuelprocessor 12 by a feed stream delivery system 17. Delivery system 17includes any suitable mechanism, device, or combination thereof thatdelivers the feed stream to fuel processor 12. For example, the deliverysystem may include one or more pumps that deliver the components ofstream 16 from a supply. Additionally, or alternatively, system 17 mayinclude a valve assembly adapted to regulate the flow of the componentsfrom a pressurized supply. The supplies may be located external of thefuel cell system, or may be contained within or adjacent the system.

Fuel cell stack 22 contains at least one, and typically multiple, fuelcells 24 adapted to produce an electric current from the portion of theproduct hydrogen stream 14 delivered thereto. This electric current maybe used to satisfy the energy demands, or applied load, of an associatedenergy-consuming device 25. Illustrative examples of devices 25 include,but should not be limited to, a motor vehicle, recreational vehicle,boat, tool, light or lighting assemblies, appliances (such as householdor other appliances), household, signaling or communication equipment,etc. It should be understood that device 25 is schematically illustratedin FIG. 1 and is meant to represent one or more devices or collection ofdevices that are adapted to draw electric current from the fuel cellsystem. A fuel cell stack typically includes multiple fuel cells joinedtogether between common end plates 23, which contain fluiddelivery/removal conduits (not shown). Examples of suitable fuel cellsinclude proton exchange membrane (PEM) fuel cells and alkaline fuelcells. Fuel cell stack 22 may receive all of product hydrogen stream 14.Some or all of stream 14 may additionally, or alternatively, bedelivered, via a suitable conduit, for use in another hydrogen-consumingprocess, burned for fuel or heat, or stored for later use.

Fuel processor 12 is any suitable device that produces hydrogen gas.Preferably, the fuel processor is adapted to produce substantially purehydrogen gas, and even more preferably, the fuel processor is adapted toproduce pure hydrogen gas. For the purposes of the present invention,substantially pure hydrogen gas is greater than 90% pure, preferablygreater than 95% pure, more preferably greater than 99% pure, and evenmore preferably greater than 99.5% pure. Suitable fuel processors aredisclosed in U.S. Pat. Nos. 5,997,594, 5,861,137, and 6,221,117, andU.S. patent application Ser. No. 09/802,361, which was filed on Mar. 8,2000 and is entitled “Fuel Processor and Systems and Devices Containingthe Same,” each of which is incorporated by reference in its entiretyfor all purposes.

An example of a suitable fuel processor 12 is a steam reformer. Anexample of a suitable steam reformer is shown in FIG. 3 and indicatedgenerally at 30. Reformer 30 includes a reforming, orhydrogen-producing, region 32 that includes a steam reforming catalyst34. Alternatively, reformer 30 may be an autothermal reformer thatincludes an autothermal reforming catalyst. In reforming region 32, areformate stream 36 is produced from the water and carbon-containingfeedstock forming feed stream 16. The reformate stream typicallycontains hydrogen gas and impurities, and therefore is delivered to aseparation region, or purification region, 38, where the hydrogen gas ispurified. In separation region 38, the hydrogen-containing stream isseparated into one or more byproduct streams, which are collectivelyillustrated at 40, and a hydrogen-rich stream 42 by any suitablepressure-driven separation process. In FIG. 3, hydrogen-rich stream 42is shown forming product hydrogen stream 14. Separation region 38includes a membrane module 44 according to the present invention andcontains one or more hydrogen-selective membranes 46. Membrane module 44is discussed and illustrated in more detail subsequently.

Reformer 30 may, but does not necessarily, further include a polishingregion 48, such as shown in FIG. 4. Polishing region 48 receiveshydrogen-rich stream 42 from separation region 38 and further purifiesthe stream by reducing the concentration of, or removing, selectedcompositions therein. For example, when stream 42 is intended for use ina fuel cell stack, such as stack 22, compositions that may damage thefuel cell stack, such as carbon monoxide and carbon dioxide, may beremoved from the hydrogen-rich stream. The concentration of carbonmonoxide should be less than 10 ppm (parts per million) to prevent thecontrol system from isolating the fuel cell stack. Preferably, thesystem limits the concentration of carbon monoxide to less than 5 ppm,and even more preferably, to less than 1 ppm. The concentration ofcarbon dioxide may be greater than that of carbon monoxide. For example,concentrations of less than 25% carbon dioxide may be acceptable.Preferably, the concentration is less than 10%, even more preferably,less than 1%. Especially preferred concentrations are less than 50 ppm.It should be understood that the acceptable minimum concentrationspresented herein are illustrative examples, and that concentrationsother than those presented herein may be used and are within the scopeof the present invention. For example, particular users or manufacturersmay require minimum or maximum concentration levels or ranges that aredifferent than those identified herein.

Region 48 includes any suitable structure for removing or reducing theconcentration of the selected compositions in stream 42. For example,when the product stream is intended for use in a PEM fuel cell stack orother device that will be damaged if the stream contains more thandetermined concentrations of carbon monoxide or carbon dioxide, it maybe desirable to include at least one methanation catalyst bed 50. Bed 50converts carbon monoxide and carbon dioxide into methane and water, bothof which will not damage a PEM fuel cell stack. Polishing region 48 mayalso include another hydrogen-producing device 52, such as anotherreforming catalyst bed, to convert any unreacted feedstock into hydrogengas. In such an embodiment, it is preferable that the second reformingcatalyst bed is upstream from the methanation catalyst bed so as not toreintroduce carbon dioxide or carbon monoxide downstream of themethanation catalyst bed.

Steam reformers typically operate at temperatures in the range of 200°C. and 700° C., and at pressures in the range of 50 psi and 1000 psi,although temperatures outside of this range are within the scope of theinvention, such as depending upon the particular type and configurationof fuel processor being used. Any suitable heating mechanism or devicemay be used to provide this heat, such as a heater, burner, combustioncatalyst, or the like. The heating assembly may be external the fuelprocessor or may form a combustion chamber that forms part of the fuelprocessor. The fuel for the heating assembly may be provided by the fuelprocessing system, or fuel cell system, by an external source, or both.

In FIGS. 3 and 4, reformer 30 is shown including a shell 31 in which theabove-described components are contained. Shell 31, which also may bereferred to as a housing, enables the fuel processor, such as reformer30, to be moved as a unit. It also protects the components of the fuelprocessor from damage by providing a protective enclosure and reducesthe heating demand of the fuel processor because the components of thefuel processor may be heated as a unit. Shell 31 may, but does notnecessarily, include insulating material 33, such as a solid insulatingmaterial, blanket insulating material, or an air-filled cavity. It iswithin the scope of the invention, however, that the reformer may beformed without a housing or shell. When reformer 30 includes insulatingmaterial 33, the insulating material may be internal the shell, externalthe shell, or both. When the insulating material is external a shellcontaining the above-described reforming, separation and/or polishingregions, the fuel processor may further include an outer cover or jacketexternal the insulation.

It is further within the scope of the invention that one or more of thecomponents may either extend beyond the shell or be located external atleast shell 31. For example, and as schematically illustrated in FIG. 4,polishing region 48 may be external shell 31 and/or a portion ofreforming region 32 may extend beyond the shell.

Although fuel processor 12, feed stream delivery system 17, fuel cellstack 22 and energy-consuming device 25 may all be formed from one ormore discrete components, it is also within the scope of the inventionthat two or more of these devices may be integrated, combined orotherwise assembled within an external housing or body. For example, afuel processor and feed stream delivery system may be combined toprovide a hydrogen-producing device with an on-board, or integrated,feed stream delivery system, such as schematically illustrated at 26 inFIG. 1. Similarly, a fuel cell stack may be added to provide anenergy-generating device with an integrated feed stream delivery system,such as schematically illustrated at 27 in FIG. 1.

Fuel cell system 10 may additionally be combined with anenergy-consuming device, such as device 25, to provide the device withan integrated, or on-board, energy source. For example, the body of sucha device is schematically illustrated in FIG. 1 at 28. Examples of suchdevices include a motor vehicle, such as a recreational vehicle,automobile, boat or other seacraft, and the like, a dwelling, such as ahouse, apartment, duplex, apartment complex, office, store or the like,or a self-contained equipment, such as an appliance, light, tool,microwave relay station, transmitting assembly, remote signaling orcommunication equipment, etc.

It is within the scope of the invention that the above-described fuelprocessor 12 may be used independent of a fuel cell stack. In such anembodiment, the system may be referred to as a fuel processing system,and it may be used to provide a supply of pure or substantially purehydrogen to a hydrogen-consuming device, such as a burner for heating,cooking or other applications. Similar to the above discussion aboutintegrating the fuel cell system with an energy-consuming device, thefuel processor and hydrogen-consuming device may be combined, orintegrated.

It is also within the scope of the present invention that the membranemodules disclosed herein may be used as a hydrogen purifier independentof a fuel processor or fuel cell stack. An example of a membrane module44 configured for use as a hydrogen-purifier is schematicallyillustrated in FIG. 5 and generally indicated at 60. As shown, a mixedgas stream 61 containing hydrogen gas 62 and other gases 63 is deliveredto purifier 60, which contains a membrane module 44 constructedaccording to the present invention. The membrane module contains atleast one hydrogen-selective membrane 46, and separates the mixed gasstream into a product stream 64 containing at least substantiallyhydrogen gas and a byproduct stream 65 containing at least substantiallythe other gases. Another way to describe the purifier is that theproduct stream contains at least a substantial portion of the hydrogengas in the mixed gas stream and that the byproduct stream contains atleast a substantial portion of the other gases. Similar to the fuelprocessors and fuel cell systems discussed above, purifier 60 may beintegrated with a hydrogen-producing device to provide ahydrogen-producing device with an integrated hydrogen purifier and/orwith a hydrogen-consuming device to provide a hydrogen-consuming devicewith an integrated hydrogen purifier.

It should be understood that the hydrogen purity of the product stream,the hydrogen content of the byproduct stream, the percentage of hydrogenfrom the mixed gas stream that forms the byproduct stream, and similarcompositions of the product and byproduct streams may be selectivelyvaried depending upon the construction of the membrane module and/or theoperating conditions within which the membrane module is used. Forexample, the compositions of the product and byproduct streams may varyat least partially in response to at least the following factors: thetemperature of the membrane module, the pressure of the membrane module,the composition of the hydrogen-selective membrane, the state of wear ofthe hydrogen-selective membrane, the thickness of the hydrogen-selectivemembrane, the composition of the mixed gas stream, the number ofhydrogen-selective membranes used in the membrane module, and the numberof sequential membranes through which the mixed gas, product and/orbyproduct streams may pass.

As discussed, a suitable structure for use in separation region 38 is amembrane module 44, which contains one or more hydrogen-permeable andhydrogen-selective membranes 46. The membranes may be formed of anyhydrogen-selective material suitable for use in the operatingenvironment and conditions in which the membrane module is operated,such as in a purifier, fuel processor or the like. Examples of suitablematerials for membranes 46 are palladium and palladium alloys, andespecially thin films of such metals and metal alloys. Palladium alloyshave proven particularly effective, especially palladium with 35 wt % to45 wt % copper, such as palladium with approximately 40 wt % copper.These membranes are typically formed from a thin foil that isapproximately 0.001 inches thick. It is within the scope of the presentinvention, however, that the membranes may be formed fromhydrogen-selective metals and metal alloys other than those discussedabove and that the membranes may have thicknesses that are larger orsmaller than discussed above. For example, the membrane may be madethinner, with commensurate increase in hydrogen flux. Suitablemechanisms for reducing the thickness of the membrane include rolling,sputtering and etching. A suitable etching process is disclosed in U.S.Pat. No. 6,152,995, the complete disclosure of which is herebyincorporated by reference.

The hydrogen-permeable membranes may be arranged in pairs around acommon permeate channel to form a membrane envelope, as is disclosed inthe incorporated patent applications and as schematically illustrated inFIG. 6 at 66. In such a configuration, the membrane pairs may bereferred to as a membrane envelope, in that they define a commonpermeate channel, or harvesting conduit, through which the permeated gasmay be collected and removed to form hydrogen-rich stream 42 (or producthydrogen stream 14 or purified hydrogen stream 64, depending on theparticular implementation of the membrane module).

It should be understood that the membrane pairs may take a variety ofsuitable shapes, such as planar envelopes and tubular envelopes.Similarly, the membranes may be independently supported, such as withrespect to an end plate or around a central passage. For purposes ofillustration, the following description and associated illustrationswill describe the membrane module as including one or more membraneenvelopes 66. It should be understood that the membranes forming theenvelope may be two separate membranes, or may be a single membranefolded, rolled or otherwise configured to define two membrane regions,or surfaces, 67 with permeate faces 68 that are oriented toward eachother to define a conduit 69 therebetween from which the permeate gasmay be collected and withdrawn.

To support the membranes against high feed pressures, a support, orscreen structure, 70 is used. Screen structure 70 provides support tothe hydrogen-selective membranes, and more particularly includessurfaces 71 that against which the permeate sides 68 of the membranesare supported. Screen structure 70 also defines harvesting conduit 69,through which permeated gas may flow both transverse and parallel to thesurface of the membrane through which the gas passes, such asschematically illustrated in FIG. 6. The permeate gas, which is at leastsubstantially pure hydrogen gas, may then be harvested or otherwisewithdrawn from the membrane module, such as to form streams 42, 64,and/or 14. Because the membranes lie against the screen structure, it ispreferable that the screen structure does not obstruct the flow of gasthrough the hydrogen-selective membrane. The gas that does not passthrough the membranes forms one or more byproduct streams, asschematically illustrated in FIG. 6.

To reiterate, the membrane module discussed herein may include one ormore membrane envelopes 66, typically along with suitable input andoutput ports through which the mixed gas stream, such as reformatestream 36 or mixed gas stream 61, is delivered to the membrane moduleand from which the hydrogen-rich and byproduct streams are removed. Insome embodiments, the membrane module may include a plurality ofmembrane envelopes. When the membrane module includes a plurality ofmembrane envelopes, the module may include fluid conduitsinterconnecting the envelopes, such as to deliver a mixed gas streamthereto, to withdraw purified hydrogen gas therefrom, and/or to withdrawthe gas that does not pass through the membranes from the membranemodule. When the membrane module includes a plurality of membraneenvelopes, the permeate stream, byproduct stream, or both, from a firstmembrane envelope may be sent to another membrane envelope for furtherpurification.

An embodiment of a suitable screen structure 70 is shown in FIGS. 7 and8 and generally indicated at 72. Screen structure 72 includes pluralscreen members 73. In the illustrated embodiment, the screen membersinclude a coarse mesh screen 74 sandwiched between fine mesh screens 76.It should be understood that the terms “fine” and “coarse” are relativeterms. Preferably, the outer screen members are selected to supportmembranes 46 without piercing the membranes and without havingsufficient apertures, edges or other projections that may pierce, weakenor otherwise damage the membrane under the operation conditions withwhich the membrane module is used. Because the screen structure needs toprovide for flow of the permeated gas generally parallel to themembranes, it is preferable to use a relatively coarser inner screenmember to provide for enhanced parallel flow conduits. In other words,the finer mesh screens provide better protection for the membranes,while the coarser mesh screen provides better flow generally parallel tothe membranes.

According to the method of the present invention, an adhesive, such as acontact adhesive, is used to secure membranes 46 to the screen structureduring fabrication. An example of a suitable adhesive is sold by 3Munder the trade name SUPER 77. An adhesive may additionally oralternatively be used to adhere the fine mesh screens to coarse meshscreen 74 during assembly. In FIG. 7, reference numerals 78 and 80 areused to indicate respectively adhesive joining membrane 46 with screenstructure 70 and individual screen members 73. It should be understoodthat adhesives 78 and 80 may have the same or different compositions,thicknesses and/or application methods.

The use of adhesive 78 allows the sandwiched screen structure to beretained as a unit in a selected configuration, such as the flat, planarconfiguration shown in FIGS. 7 and 8. The use of adhesive 80 allows thethin membranes to be firmly attached to the screen structure without anywrinkles in the membrane. It is important that these components be heldflat and in close contact during assembly of the membrane module. If themembrane buckles, or if the screen structure buckles, then a wrinklewill form in the membrane during use. Similarly, if the membranes areimproperly positioned relative to the screen structure, wrinkles mayalso occur, such as when the membrane module is pressurized. As pointedout above, wrinkles in the membrane lead to stress fractures and fatiguefractures, causing failure of the membrane module and contamination ofthe purified gas stream.

In practice, a light coating of contact adhesive 78 is sprayed orotherwise applied to the two opposing major surfaces of the coarse meshscreen 74 and then fine mesh screens 76 are attached, one to each majorsurface of the coarse screen. Adhesive 78 holds screen structure 72together. Alternatively, the adhesive may be applied to screens 76instead of being applied to the coarse screen. Similarly, adhesive 80 isapplied between the corresponding surfaces of the fine mesh screens andhydrogen-selective membranes 46 may then be adhesively secured to theopposed surfaces of the fine mesh screens. As discussed herein, theadhesive is at least substantially, or completely, removed afterfabrication of the membrane envelope and/or membrane modules so as tonot interfere with the operation of the membrane envelopes.

It is within the scope of the invention that the screen members may beof similar or the same construction, and that more or less screenmembers may be used. It is also within the scope of the invention thatany suitable supporting medium that enables permeated gas to flow in theharvesting conduit generally parallel and transverse to the membranesmay be used. For example, porous ceramics, porous carbon, porous metal,ceramic foam, carbon foam, and metal foam may be used to form screenstructure 70, either alone, or in combination with one or more screenmembers 73. As another example, fine mesh screens 76 may be formed fromexpanded metal instead of a woven mesh material. Preferably, screenstructure 70 is formed from a corrosion-resistant material that will notimpair the operation of the membrane module and devices with which themembrane module is used. Examples of suitable materials for metallicscreen members include stainless steels, titanium and alloys thereof,zirconium and alloys thereof, corrosion-resistant alloys, includingInconel™ alloys, such as 800H™, and Hastelloy™ alloys, and alloys ofcopper and nickel, such as Monel™.

An example of an expanded metal screen member is shown in FIG. 9 andgenerally indicated at 82. Expanded metal sheets include a latticework83 of metal that defines a plurality of apertures 84 through whichpermeated gas may flow. Although other processes may be used, expandedmetal sheets may be formed from scoring a sheet of metal and thenstretching the metal to provide apertures, such as apertures 84 at thescores. It should be understood that the expanded metal screen memberhas been schematically illustrated in FIG. 9, and that the actual shapeof the apertures may vary and will often have shapes that generallyresemble diamonds, parallelograms or other geometric shapes, for exampleas shown in FIG. 12. The sheet may also include a solid perimeter region86, which is advantageous because it is free from projections, burrs, orother wire ends that may be present in woven mesh screen members andwhich may pierce or otherwise damage the hydrogen-selective membranes.Although only a portion of expanded metal screen member 82 is shown inFIG. 9, the perimeter region 86 of the screen member may extend all theway around the screen member. Alternatively, only the perimeter regionsthat contact membranes 46 may be solid surfaces.

All of the foregoing metallic screen compositions may include a coating85 on the surface against which the permeate sides of the membranes aresupported (such as shown in FIG. 8). Examples of suitable coatingsinclude aluminum oxide, tungsten carbide, tungsten nitride, titaniumcarbide, titanium nitride, and mixtures thereof. These coatings aregenerally characterized as being thermodynamically stable with respectto decomposition in the presence of hydrogen. Suitable coatings areformed from materials, such as oxides, nitrides, carbides, orintermetallic compounds, that can be applied as a coating and which arethermodynamically stable with respect to decomposition in the presenceof hydrogen under the operating parameters (temperature, pressure, etc.)in which the membrane module will be subjected. Alternatively, thecoating may be applied to an expanded metal screen member that is usedin place of a fine mesh screen, in which case the coating would beapplied to at least the surface of the expanded mesh that will contactthe hydrogen-selective membrane 46. Suitable methods for applying suchcoatings to the screen or expanded metal screen member include chemicalvapor deposition, sputtering, thermal evaporation, thermal spraying,and, in the case of at least aluminum oxide, deposition of the metal(e.g., aluminum) followed by oxidation of the metal to give aluminumoxide. In at least some embodiments, the coatings may be described aspreventing intermetallic diffusion between the hydrogen-selectivemembranes and the screen structure.

Preferably, the screen structure and membranes are incorporated into amembrane module that includes frame members 88 that are adapted to seal,support and/or interconnect the membrane envelopes for use in fuelprocessing systems, gas purification systems, and the like. Fine meshmetal screen 76 fits within permeate frame 90. Expanded metal screenmember 86 may either fit within permeate frame 90 or extend at leastpartially over the surface of permeate frame 90. Examples of suitableframe members 88 include supporting frames and/or gaskets. These frames,gaskets or other support structures may also define, at least in part,the fluid conduits that interconnect the membrane envelopes in anembodiment of membrane module 44 that contains two or more membraneenvelopes. Examples of suitable gaskets are flexible graphite gaskets,although other materials may be used, such as depending upon theoperating conditions in which a particular membrane module is used.

An example of a membrane envelope 66 that includes frame members 88 isshown in FIG. 12. As shown, screen structure 70 is placed in a permeateframe 90 that forms a portion of membrane module 44. The screenstructure and frame 90 may collectively be referred to as a screen frameor permeate frame 91. Permeate gaskets 92 and 92′ are attached topermeate frame 90, preferably but not necessarily, by using another thinapplication of adhesive. Next, membranes 46 are attached to screenstructure 70 using a thin application of adhesive, such as by sprayingor otherwise applying the adhesive to either or both of the membraneand/or screen structure. Care should be taken to ensure that themembranes are flat and firmly attached to the corresponding screenmember 73. Finally, feed plates, or gaskets, 94 and 94′ are optionallyattached, such as by using another thin application of adhesive. Theresulting membrane assembly is then stacked with feed, or end, plates toform membrane module 44. Optionally, two or more membrane envelopes maybe stacked between the end plates.

Optionally, each membrane 46 may be fixed to a frame 104, such as ametal frame and such as shown in FIG. 11. If so, the membrane is fixedto the frame, for instance by ultrasonic welding or another suitableattachment mechanism, and the membrane-frame assembly is then attachedto screen structure 70 using adhesive. Other examples of attachmentmechanisms achieve gas-tight seals between plates forming membraneenvelope 66, as well as between the membrane envelopes, include one ormore of brazing, gasketing, and welding. The membrane and attached framemay collectively be referred to as a membrane plate 96.

For purposes of illustration, the geometry of fluid flow throughmembrane envelope 66 is described with respect to the embodiment ofenvelope 66 shown in FIG. 10. As shown, a mixed gas stream, such asreformate stream 36, is delivered to the membrane envelope and contactsthe outer surfaces 97 of membranes 46. The hydrogen gas that permeatesthrough the membranes enters harvesting conduit 69, which is formedbetween the permeate faces 68 of the membranes. The harvesting conduitis in fluid communication with conduits 100 through which the permeatestream may be withdrawn from the membrane envelope. The portion of themixed gas stream that does not pass through the membranes flows to aconduit 98 through which this gas may be withdrawn as byproduct stream40. In FIG. 10, a single byproduct conduit 98 is shown, while in FIG. 11a pair of conduits 98 and 102 are shown to illustrate that any of theconduits described herein may alternatively include more than one fluidpassage. It should be understood that the arrows used to indicate theflow of streams 40 and 42 have been schematically illustrated, and thatthe direction of flow through conduits 98, 100 and 102 may vary, such asdepending upon the configuration of a particular membrane module. Alsoshown in FIG. 10 are other illustrative examples of frame members 88,and in FIG. 11 frame members 88 and membrane plates 96 are shown.

In FIG. 12, another example of a suitable membrane envelope 66 is shown.For purposes of illustration, envelope 66 is shown having a generallyrectangular form. The envelope of FIG. 12 also provides another exampleof a membrane envelope having a pair of byproduct conduits 98 and 102and a pair of hydrogen conduits 100. As shown, envelope 66 includesgaskets or spacer plates 94 as the outer most plates in the stack.Generally, each of spacer plates includes a frame 106 that defines aninner open region 108. Each inner open region 108 couples laterally toconduits 98 and 102. Conduits 100, however, are closed relative to openregion 108, thereby isolating the hydrogen-rich stream 42. Membraneplates 96 lie adjacent and interior to plates 94. Membrane plates 96each include as a central portion thereof a hydrogen-selective membrane46, which may be secured to an outer frame 104 that is shown forpurposes of illustration. In plates 96, all of the ports are closedrelative to membrane 46. Each membrane lies adjacent to a correspondingone of open regions 108, i.e., adjacent to the flow of mixed gasarriving to the envelope. This provides opportunity for hydrogen to passthrough the membrane, with the remaining gases, i.e., the gases formingbyproduct stream 40, leaving open region 108 through conduit 102. Screenplate 91 lies intermediate membrane plates 96, i.e., on the interior orpermeate side of each of membranes 46. Screen plate 91 includes a screenstructure 70. Conduits 98 and 102 are closed relative to the centralregion of screen plate 91, thereby isolating the byproduct stream 40 andthe reformate-rich flow 36 from hydrogen-rich stream 42. Conduits 100are open to the interior region of screen plate 91. Hydrogen, havingpassed through the adjoining membranes 46, travels along and throughscreen structure 70 to conduits 100 and eventually to an output port asthe hydrogen-rich stream 42.

As discussed, membrane module 44 may include one or more membraneenvelopes in which the membranes have been adhesively bonded to thescreen structure, and/or in which the screen structure includes two ormore screen members 83 that are adhesively bonded together. Typically,the membrane module further includes end plates having input and outputports through which the mixed gas, product (or hydrogen-rich) andbyproduct streams are removed from the membrane module. An example of asuitable membrane module is shown in FIG. 13 in the form of a platemembrane module. As shown, the module contains end plates 110 betweenwhich one or more membrane envelopes 66 are contained. In theillustrated embodiment, three membrane envelopes are shown for purposesof illustration, but it should be understood that more or less envelopesmay be used. The membrane envelopes are in fluid communication with atleast one of the end plates, through which the mixed gas stream isdelivered and from which the byproduct 40 and hydrogen-rich 42 streamsare removed.

As shown in the illustrative embodiment of FIG. 13, one of the endplates contains a reformate input port 112 for a mixed gas stream, suchas reformate stream 36 or any of the other feeds to the membrane modulesdiscussed herein. The end plates further include a pair of exit ports114 for permeate, or hydrogen-rich, stream 42 and an exit port 116 forbyproduct stream 40. It should be understood that the number and sizingof the ports for each stream may vary, and that at least one of theports may be contained on the other end plate or elsewhere on themembrane module, such as on a housing 118 between the end plates, suchas shown in FIG. 15. As shown, the membrane envelopes include conduits98, 100 and 102 that establish fluid communication with the input andexit ports and between the membrane envelopes. When membrane envelopes66 are stacked, these various ports align and provide fluid conduits.

In operation, reformate gas is introduced to the membrane module throughport 112 and is delivered to the membrane envelopes. Hydrogen gas thatpasses through the hydrogen-selective membranes 46 flows to conduits 100and is removed from the membrane module through ports 114. The rest ofthe reformate gases, namely the portion that does not pass through thehydrogen-selective membranes, flows to conduit 102 and is removed fromthe membrane module as byproduct stream 40 through port 116.

It should be understood that the geometry of the frame members, gaskets,membranes and screen members shown in the FIGS. 7-13 are provided asillustrative examples, and it should be understood that these componentsmay be of any suitable shape. For example, illustrations of circular andrectangular plate membrane envelopes are illustrated in FIGS. 10-13 forpurposes of illustration. Other shapes, and other configurations, suchas tubular configurations, are also within the scope of the presentinvention. Similarly, the configuration and orientation of the passagesthrough the gaskets and plates may vary, depending upon the particularapplication with which the membrane module will be used.

Membrane modules containing the palladium alloy membranes that areadhesively bonded to screen structure 70 preferably are subjected tooxidizing conditions prior to initial operation of the membrane moduleto remove the adhesive. If adhesive is not fully removed prior tooperation, the carbon residue from the adhesive can alloy with thepalladium-alloy membrane and cause a decline in hydrogen permeability.In extreme cases, carbon alloying with the palladium-alloy membrane canform a brittle alloy that physically fails under operating conditions.

The objective of the oxidative conditioning is to burn out the adhesivewithout excessively oxidizing the palladium-alloy membrane. One set ofsuitable conditions using the above membrane compositions and adhesiveis to heat the membrane module to 200° C. while passing air over boththe feed side and the permeate side of the membrane. A preferred methodis to heat the membrane module to 200° C. while the feed side ispressurized to a pressure greater than the permeate side of themembranes using a slow purge of air (>1 mL/min). Pressures in the rangeof approximately 50 psig to approximately 200 psig have proveneffective. Air at approximately ambient pressure is passed over thepermeate side of the membrane at a rate >1 mL/min. These conditions aremaintained for approximately 15-25 hours. Then the temperature isincreased to 400° C., while maintaining air pressure and flow rate overthe feed and permeate sides of the membranes. The temperature is held at400° C. for approximately 2-5 hours. After completing this oxidativeconditioning of the membrane module, the adhesive has been burned out ofthe membrane module and the module is ready to accept ahydrogen-containing feed stream to be purified. Experiments have shownthat these methods result in membrane modules containing membranes thatare free of wrinkles and without excessive carbon contamination.

It should be understood that the conditions described above werepresented to provide an illustrative example and that the operatingconditions may vary. For example, different conditions may be usedbecause of such factors as different membrane compositions, differentmembrane thicknesses, and different adhesives. Similarly, the inventedmethod using an adhesive to secure hydrogen-selective membranes on oneor more support screens may be used with purification assemblies otherthan the fuel processing assemblies described herein and in theincorporated patent applications.

An example of a fuel processor 12 containing a membrane module 44according to the present invention is shown in FIG. 14. In theillustrated embodiment, fuel processor 12 is shown as a steam reformer30 that contains reforming catalyst 34. Alternatively, reformer 30 maybe an autothermal reformer that contains an autothermal reformingcatalyst bed. It should be understood that fuel processor 12 may be anydevice adapted to produce hydrogen gas, such as those discussed herein.

In the embodiment of steam reformer 30 shown in FIG. 14, a feed stream16 is delivered to a vaporization region 150, which as shown contains avaporization coil 151 in which the feed stream is vaporized. For a steamreformer, a suitable feed stream includes water and a carbon-containingfeedstock, such as one or more alcohols or hydrocarbons. When thecarbon-containing feedstock is miscible with water, the feedstock andwater may be mixed and then vaporized. When the carbon-containingfeedstock is not miscible with water, the water is typically vaporizedand then mixed with the carbon-containing feedstock. In the illustratedembodiment, vaporization coil 151 is contained within the shell 31 ofthe reformer. It is within the scope of the invention that thevaporization region (and coil) may be located external the shell of thefuel processor, such as extending around the shell or otherwise locatedoutside of the shell.

The vaporized feed stream is then delivered to hydrogen-producing region32, which in the context of a reformer, contains at least one reformingcatalyst bed. The reformate stream, which is a mixed gas streamcontaining hydrogen gas and other gases, 36 is then delivered tomembrane module 44, which separates the mixed gas stream intohydrogen-rich stream 42 and byproduct stream 40, as discussed above. Theillustrated reformer demonstrates that the byproduct stream may be usedto provide some or all of the fuel for the reformer's heating assembly152. Heating assembly 152 includes a heating element 153, which in theillustrated embodiment takes the form of a spark plug. Examples of othersuitable heating elements include glow plugs, pilot lights, combustioncatalysts, resistance heaters, and combinations thereof, such as a glowplug in combination with a combustion catalyst.

Heating assembly 152 consumes a fuel stream 154, which may be acombustible fuel stream or an electric current, depending upon the typeof heating element used in the heating assembly. In the illustratedembodiment, the heating assembly forms part of a combustion chamber, orregion, 155, and the fuel stream includes a combustible fuel and airfrom an air stream 156. The fuel may come from an external source, suchas schematically illustrated at 157, or may be at least partially formedfrom the byproduct stream 40 from membrane module 44. It is within thescope of the invention that at least a portion of the fuel stream mayalso be formed from product hydrogen stream 14. In the illustratedembodiment, the exhaust from combustion region 155 flows through heatingconduits 158 in reforming region 32 to provide additional heating to thereforming region. Conduits 158 may take a variety of forms, includingfinned tubes and spirals, to provide sufficient surface area anddesirable uniform distribution of heat throughout reforming region 32.

In FIG. 15, another illustrative example of a steam reformer containinga membrane module 44 constructed according to the present invention isshown. As shown, the reforming region includes a plurality of reformingcatalyst tubes 162 that contain reforming catalyst 34. The vaporizedfeed stream from vaporization region 150 is delivered to tubes 162 via amanifold 172 that distributes the feed stream between reforming catalysttubes. As shown in dashed lines in FIG. 15, the manifold mayalternatively be located external shell 31 to enable access to themanifold from external the shell, such as to adjust the relativedistribution of the vaporized feed stream between the reforming catalysttubes. Similarly, portions 160 of the reforming catalyst tubes are alsoshown extending beyond shell 31.

The steam reformer of FIG. 15 also provides an example of a fuelprocessor 12 in which the byproduct stream may be either used as aportion of fuel stream 154 for combustion region 155, vented (such asthrough pressure-relief valve assembly 164), or delivered through fluidconduit 166 for storage or use outside of fuel processor 12. Also shownin FIG. 15 are flow regulators 168 for heat produced by heating assembly152 in combustion region 155. In the illustrated embodiment, regulators168 take the form of apertures in a combustion manifold 170. Theapertures regulate the path along which combustion exhaust travels fromcombustion region 155 and through reforming region 32. Examples ofsuitable placement of the apertures include one or more apertures distalheating assembly 152, and a plurality of apertures distributed along thelength of manifold 170. When a distribution of spaced-apart apertures isused, the apertures may be evenly spaced, or the openings may be moreprevalent distal the burner. Similarly, the size of the apertures may beuniform, or may vary, such as using larger apertures away from heatingassembly 152.

It should be understood that the steam reformers shown in FIGS. 14 and15 are shown and described for purposes of illustration and should notbe construed as providing exclusive embodiments of fuel processors orsteam reformers with which the invented membrane modules may be used.Instead, the structure and components of reformers and fuel processorscontaining membrane modules according to the invention may vary.

As discussed above, membranes 46 may be formed from a variety ofmaterials and by a variety of methods, including a method that involvesetching a membrane to reduce the thickness of at least a portion thereofto increase the hydrogen-permeability of the membrane. Although notrequired for the above-described membrane envelopes, modules, hydrogenpurifiers, and the like, etching a hydrogen-permeable (and selective)membrane to reduce the thickness of at least a portion thereof has beendemonstrated to effectively increase the hydrogen flux through themembrane compared to a membrane that has not been etched.

An unetched hydrogen-permeable membrane is shown in FIG. 16 andindicated generally at 210. As discussed, membrane 210 may, but is notrequired to, represent any of the previously described and illustratedmembranes 46. Similarly, the subsequently described etched membrane 230may, but is not required to, take the place of any of the previouslydescribed, illustrated and/or incorporated membranes. Membrane 210includes a pair of generally opposed surfaces 212 and 214 and an edge216 joining the perimeters of the surfaces. Each surface 212 and 214includes an outer edge region 218 that surrounds a central region 220.Membrane 210 is typically roll formed and, as shown, has a generallyrectangular, sheet-like configuration with a constant thickness. Itshould be understood that membrane 210 may have any geometric orirregular shape, such as by cutting the formed membrane into a desiredshape based on user preferences or application requirements. It iswithin the scope of the invention that any suitable method for formingmembrane 210 may be used. For example, membrane 210 may also be formedfrom such processes as electro deposition, sputtering or vapordeposition.

In FIG. 17, membrane 210 is shown in cross-section, and it can be seenthat the thickness 222 of the membrane measured between the centralregions is the same as the thickness 224 measured between the edgeregions. In the figures, it should be understood that the thicknesses ofthe membranes and subsequently described absorbent media and frame havebeen exaggerated for purposes of illustration. Typically,hydrogen-permeable membranes have thicknesses less than approximately 50microns, although the disclosed etching process may be used with thickermembranes.

Also shown in FIG. 17 is a portion of a frame 226, which may be securedto the membrane, such as around a portion or the entire edge region 218.Frame 226 is formed from a more durable material than the membrane andprovides a support structure for the membrane. Frame 226 may be securedto one or both surfaces of the membrane. It should be understood thatthe invented membrane may be formed without frame 226. In anothervariation, frame 226 may take the form of a compressible gasket that issecured to the membrane, such as with an adhesive or other suitablestructure or process. Compressible gaskets are used to form gas-tightseals around and/or between the membranes.

In use, membrane 210 provides a mechanism for removing hydrogen frommixtures of gases because it selectively allows hydrogen to permeatethrough the membrane. The flowrate, or flux, of hydrogen throughmembrane 210 typically is accelerated by providing a pressuredifferential between a mixed gaseous mixture on one side of themembrane, and the side of the membrane to which hydrogen migrates, withthe mixture side of the membrane being at a higher pressure than theother side.

Membrane 210 is formed of a hydrogen-permeable metal or metal alloy,such as palladium or a palladium alloy. An example of such an alloy iscomprised of 60 wt % palladium and 40 wt % copper (generally known asPd-40Cu). Because palladium and palladium alloys are expensive, thethickness of the membrane should be minimal; i.e., as thin as possiblewithout introducing an excessive number of holes in the membrane. Holesin the membrane are not desired because holes allow all gaseouscomponents, including impurities, to pass through the membrane, therebycounteracting the hydrogen-selectivity of the membrane.

It is known to roll form hydrogen-permeable metal membranes, such asmembrane 210, to be very thin, such as with thicknesses of less thanapproximately 50 microns, and more commonly with thicknesses ofapproximately 25 microns. The flux through a hydrogen-permeable metalmembrane is inversely proportional to the membrane thickness. Therefore,by decreasing the thickness of the membrane, it is expected that theflux through the membrane will increase, and vice versa. In Table 1,below, the expected flux of hydrogen through various thicknesses ofPd-40Cu membranes is shown.

TABLE 1 Expected hydrogen flux through Pd-40Cu membranes at 400° C. and100 psig hydrogen feed, permeate hydrogen at ambient pressure. MembraneThickness Expected Hydrogen Flux 25 micron  60 mL/cm² · min 17 micron 88 mL/cm² · min 15 micron 100 mL/cm² · min

Besides the increase in flux obtained by decreasing the thickness of themembrane, the cost to obtain the membrane also increases as themembrane's thickness is reduced. Also, as the thickness of a membranedecreases, the membrane becomes more fragile and difficult to handlewithout damaging.

Through the etching process, or method, of the present invention,discussed in more detail subsequently, the thickness of a portion of themembrane, such as central portion 220, may be selectively reduced, whileleaving the remaining portion of the membrane, such as edge region 218,at its original thickness. Therefore, greater flux is obtained in thethinner etched region, while leaving a thicker, more durable edge regionthat bounds the central region and thereby provides support to themembrane.

For example, an etched membrane prepared according to an etching methodof the present invention is shown in FIG. 18 and illustrated generallyat 230. Like membrane 210, membrane 230 includes a pair of generallyopposed surfaces 232 and 234 and an edge 236 joining the surfaces. Eachsurface 232 and 234 includes an outer edge region 238 that surrounds acentral region 240. Membrane 230 is formed from any of theabove-discussed hydrogen-permeable metal materials, and may have any ofthe above-discussed configurations and shapes. The etching process workseffectively on work-hardened, or non-annealed membranes. Alternatively,the membranes may be annealed prior to the etching process. Unlikemembrane 210, however, the thickness 242 of membrane 230 measuredbetween central regions 240 is less than the thickness 244 measuredbetween the edge regions, as schematically illustrated in FIG. 19.Therefore, the hydrogen flux through the central region will be greaterthan that through the edge region, as expected from the above discussionof the inversely proportional relationship between membrane thicknessand hydrogen flux.

However, an unexpected benefit of chemically etching the membrane, asdisclosed herein, is that the hydrogen flux through the etched regionexceeds that expected or measured through roll-formed membranes of equalthickness. As shown below in Table 2, the method of the presentinvention yields a hydrogen-permeable metal membrane with significantlygreater flux than unetched membranes of similar thicknesses.

TABLE 2 Hydrogen flux through etched and unetched Pd-40Cu membranes at400° C. and 100 psig hydrogen feed, permeate hydrogen at ambientpressure. Aqua regia etchant. Etching Membrane Observed Expected TimeThickness Hydrogen Flux Hydrogen Flux None 25 micron  60 mL/cm² · min 60 mL/cm² · min 2.0 mins. 17 micron  94 mL/cm² · min  88 mL/cm² · min2.5 mins. 15 micron 122 mL/cm² · min 100 mL/cm² · min

As the above table demonstrates, the invented method produceshydrogen-permeable metal membranes that permit increased hydrogenthroughput compared to unetched membranes of similar thickness byincreasing the roughness and surface area of the etched region of themembrane. Perhaps more importantly, this increase in throughput isachieved without sacrificing selectivity for hydrogen or the purity ofthe harvested hydrogen gas, which is passed through the membrane.

Increasing the surface roughness of the membrane is especiallybeneficial as the thickness of the membrane is reduced to less than 25microns, especially less than 20 microns. As the membrane thickness isreduced, the surface reaction rates governing the transport of gaseousmolecular hydrogen onto the surface of the metal membrane become moreimportant to the overall permeation rate of hydrogen across themembrane. In extreme cases in which the membrane is quite thin (lessthan approximately 15 microns) the surface reaction rates aresignificant in governing the overall permeation rate of hydrogen acrossthe membrane. Therefore, increasing the surface area increases the rateof hydrogen permeation. This contrasts with relatively thick membranes(greater than 25 microns) in which the surface reaction rates are lessimportant and the overall permeation rate of hydrogen across themembrane is governed by the bulk diffusion of hydrogen through themembrane.

Thus the etching process results in an overall reduction in thethickness of the membrane and an increase in the surface roughness (andsurface area) of the membrane. These improvements yield an increase inhydrogen flux and reduce the amount of material (e.g., palladium alloy)that is required, while still maintaining the membrane's selectivity forhydrogen.

In the invented etching process, an etchant is used to selectivelyreduce the thickness of the membrane. When the etchant removes, oretches, material from the surface of a membrane, the etchant alsoincreases the surface roughness and surface area of the membrane in theetched region.

Examples of suitable etchants are oxidizing agents and acids. Forexample, oxidizing acids such as nitric acid. Other suitable examplesare combinations of nitric acid with other acids, such as aqua regia (amixture of 25 vol % concentrated nitric acid and 75 vol % concentratedhydrochloric acid). Another specific example of an etchant well-suitedto use in the present invention is a mixture comprising 67 wt %concentrated nitric acid and 33 wt % aqueous solution of poly(vinylalcohol). A suitable method of preparing the aqueous solution ofpoly(vinyl alcohol) is to dissolve 4 wt % of poly(vinyl alcohol)(average molecular weight 124,000 to 186,000; 87% to 89% hydrolyzed;Aldrich Chemical Company, Milwaukee, Wis.) in de-ionized water. Thedisclosed examples of etchants are for illustrative purposes, and shouldnot be construed to be limiting examples. For example, the relativepercentage of acid may be increased or decreased to make the etchantrespectively more or less reactive, as desired.

In a first method of the present invention, a selected etchant isapplied to at least one of the surfaces of the membrane. Once applied,the etchant removes material from the surface of the membrane, therebyincreasing its surface roughness and reducing the thickness of themembrane in the etched region. After a defined time period, the etchantis removed. The etching process disclosed herein typically is conductedunder ambient conditions (temperature and pressure), although it shouldbe understood that the process could be conducted at elevated or reducedtemperatures and pressures as well.

The etching process is limited either by the time during which themembrane is exposed to the etchant, or by the reactive elements of theetchant. In the latter scenario, it should be understood that theetching reaction is self-limiting, in that the reaction will reach anequilibrium state in which the concentration of dissolved membrane inthe etchant solution remains relatively constant. Regardless of thelimiting factor in the process, it is important to apply a volume andconcentration of etchant for a time period that will not result in theetchant creating substantial holes in, or completely dissolving, themembrane. Preferably, no holes are created in the membrane during theetching process.

When applying the etchant to a surface of membrane 210, such as toproduce membrane 230, it is desirable to control the region of thesurface over which the etchant extends. It is also desirable to maintainan even distribution of etchant over this application region. If theapplication region of the etchant is not controlled, then the etchantmay remove material from other non-desired regions of the membrane, suchas the edge region, or may damage materials joined to the membrane, suchas an attached frame. If an even distribution of etchant is notmaintained, areas of increased etchant may have too much materialremoved, resulting in holes in the membrane. Similarly, other areas maynot have enough material removed, resulting in less than the desiredreduction in thickness and increase in flux.

To control the distribution of etchant within the desired applicationregion, an absorbent medium is placed on the membrane 210 and defines anapplication region to be etched. For example, in FIGS. 20 and 21, theabsorbent medium is generally indicated at 250 and covers applicationregion 252 of surface 212. As shown, medium 250 is sized to cover only acentral portion of surface 212, however, it should be understood thatmedium 250 may be selectively sized to define application regions of anydesired size and shape, up to the complete expanse of surface 212.Typically, however, only a central portion of each surface is treated,leaving an unetched perimeter of greater thickness than the centralregion. This unetched region, because of its greater thickness, providesstrength and support to membrane 210 while still contributing to thehydrogen permeability of the membrane.

Besides being selected to absorb the particular etchant withoutadversely reacting to the etchant or metal membrane, it is preferablethat medium 250 has a substantially uniform absorbency and diffusivityalong its length. When medium 250 absorbs and distributes the etchantuniformly along its length, it distributes the etchant evenly across theapplication region, thereby removing substantially the same amount ofmaterial across the entire application region. The benefit of this isnot only that some etchant will contact, and thereby remove materialfrom the entire application region, but also that the etchant will beuniformly distributed across the application region. Therefore, medium250 prevents too much etchant being localized in an area, which wouldresult in too much material being removed. In a region where too muchetchant is applied, the excess etchant is drawn away from that region toother areas of the medium where less etchant is applied. Similarly, in aregion where too little etchant is applied, the medium draws etchant tothat region to produce an even distribution across the medium, andthereby across the application region.

As a result, the reduction of thickness in membrane 210 will berelatively uniform across the application region, and perhaps, moreimportantly, will be reproducible regardless of the exact rate andposition at which the etchant is applied. Therefore, with the same sizeand type of medium 250 and the same volume of etchant 254, the resultingreduction in thickness should be reproducible for membranes of the samecomposition. Of course, it should be understood that etching removesmaterial from the surface of the membrane, thereby resulting in anuneven, rough surface with increased surface area over an unetchedsurface. Therefore, the exact surface topography will not be seen.However, the average thickness measured across a section of the membraneshould be reproducible. For example, in FIG. 19, the average thicknessbetween central regions 240 is indicated with dashed lines.

Because medium 250 essentially defines the bounds of application region252, medium 250 should be sized prior to placing it upon the surface tobe etched. After placing the medium in the desired position on one ofthe membrane's surfaces, such as surface 212 shown in FIG. 20, a volumeof etchant is applied. In FIG. 20, the applied volume of etchant isschematically illustrated at 254, with arrows 256 illustrating theabsorption and distribution of etchant 254 across medium 250.

The applied volume of etchant should be no more than a saturation volumeof etchant. An absorbent medium can only absorb up to a defined volumeof a particular etchant per unit of medium 250 before reaching thesaturation point of the medium. Therefore, it is important not to exceedthis saturation point. Too much applied etchant will result inunabsorbed etchant pooling on or adjacent to the medium, such as on theupper surface of the medium 250 or around the edges of the medium. Whenexcess etchant contacts the surface, it is likely to result in holes inthe membrane because more than the desired amount of material isremoved. As discussed, if these holes are numerous or large enough, theywill render the membrane unusable for hydrogen purificationapplications, with any holes lowering the purity of the hydrogen passingthrough the membrane.

Therefore, to prevent too much etchant from being applied, the volume ofetchant applied may approach, but should not exceed, the saturationvolume of the etchant.

An example of a suitable absorbent medium is a cellulosic material, suchas absorbent paper products. A particular example of an absorbent mediumthat has proven effective are single-fold paper towels manufactured bythe Kimberly Clark company. When a three inch by three inch area of sucha towel is used, approximately 2.5 mL of etchant may be applied withoutexceeding the saturation volume of that area. The capillary action ofthe cellulosic towel both absorbs the applied etchant and distributesthe etchant throughout the towel. Other paper and cellulosic materialsmay be used as well, as long as they meet the criteria defined herein.Absorbent, diffusive materials other than cellulosic materials may beused as well.

After applying the etchant to medium 250, the etchant is allowed toremove material from the application region for a determined timeperiod. This period is best determined through experimentation and willvary depending on such factors as the composition, thickness and desiredthickness of the membrane, the absorbent medium being used, thecomposition and concentration of etchant, and the temperature at whichthe etching process is conducted. After this time period has passed, themedium is removed from the membrane, and the application, or treatmentarea is rinsed with water to remove any remaining etchant. Afterrinsing, the method may be repeated to etch another surface of themembrane.

Instead of a single etching step on each surface of the membrane, avariation of the above method includes plural etching steps for eachsurface to be etched. In the first step, a more reactive, or vigorousetchant is used to remove a substantial portion of the material to beremoved. In the second step, a less reactive etchant is used to providea more controlled, even etch across the application region.

As an illustrative example, Pd-40Cu alloy foil was etched first withconcentrated nitric acid for 20-30 seconds using the absorbent mediumtechnique described above. After removing the medium and rinsing anddrying the membrane, a second etch with a mixture of 20 vol % neatethylene glycol and the balance concentrated nitric acid was performedfor between 1 and 4 minutes. Subsequent etching steps were performedwith the glycol mixture to continue to gradually reduce the thickness ofthe membrane in the application region. Results of etching Pd-40Cu foilusing this method are given in the table below.

TABLE 3 Results of etching Pd-40Cu membrane with concentrated nitricacid for 30 seconds followed by subsequent etches with concentratednitric acid diluted with 20% vol ethylene glycol. Etching SolutionEtching Time Observations None (Virgin Pd-40Cu N/A Measures 0.0013 Foil)inches thick 1) Conc. Nitric Acid 1) 30 seconds Measures 0.0008 2) 20vol % ethylene 2) 1.5 to 0.0009 inches glycol/HNO₃ minutes thick, no pinholes 1) Conc. Nitric Acid 1) 30 seconds Measures 0.0005 2) 20 vol %ethylene 2) 1.5 to 0.0006 inches glycol/HNO₃ minutes thick, no pin holes3) 20 vol % ethylene 3) 1.5 glycol/HNO₃ minutes 1) Conc. Nitric Acid 1)30 seconds Measures 0.0005 2) 20 vol % ethylene 2) 3 minutes inchesthick, no glycol/HNO₃ pin holes in membrane 1) Conc. Nitric Acid 1) 1minute Multiple pin holes 2) 20 vol % ethylene 2) 3 minutes in membraneglycol/HNO₃

Other than confining the etching solution to a desired applicationregion, another benefit of using an absorbent medium to control theplacement and distribution of the etchant is that the quantity ofetchant (or etching solution) that may be applied without oversaturatingthe medium is limited. Thus, the etching reaction may be self-limiting,depending on the choice of and composition of etchant. For instance,varying the etching time using 33.3 wt % PVA solution/66.7 wt %concentrated HNO₃ yielded the results shown in the following table.These results indicate that the volume of etchant that is applied at onetime may limit the depth of etching, so long as the etchant is not soreactive or applied in sufficient quantity to completely dissolve theapplication region.

TABLE 4 Results of etching Pd-40Cu membrane with a solution of 33.3 wt %PVA solution/66.7 wt % concentrated nitric acid. Etching TimeObservations 0 Measures 0.0013 inches thick 3 minutes Measures 0.0011inches thick 4 minutes Measures 0.0011 inches thick 5 minutes Measures0.0011 inches thick 6 minutes Measures 0.0011 inches thick 3 minutes,rinse, 3 Measures 0.0008 to 0.0009 minutes inches thick 3 minutes,rinse, 3 Measures 0.0006 inches thick, minutes, rinse, 3 multiple pinholes minutes

In a further variation of the etching method, a suitable mask may beapplied to the membrane to define the boundaries of the region to beetched. For example, in FIG. 20, instead of using absorbent medium 250to define application region 252, a non-absorbent mask could be appliedaround edge region 238. Because this mask does not absorb the etchant,it confines the etchant to an application region bounded by the mask.Following etching, the mask is removed. The mask may be applied as aliquid or it may be a film with an adhesive to bond the film to themembrane.

If the chemical etching process is not properly controlled, tiny holeswill appear in the membrane. For example, in FIG. 22 membrane 230 isshown with a hole 260 in its central region 240. Typically, the holeswill be very small, however, the size of a particular hole will dependon the concentration and quantity of etchant applied to that region, aswell as the time during which the etchant was allowed to etch materialfrom the membrane. Holes, such as hole 260, reduce the purity of thehydrogen gas harvested through the membrane, as well as the selectivityof the membrane for hydrogen. The probability of holes forming in themembrane during the etching process increases as the thickness of themembrane is reduced. Therefore, there is often a need to repair anyholes formed during the etching process.

One method for detecting any such holes is to utilize a light source toidentify holes in the membrane. By shining a light on one side of themembrane, holes are detected where light shines through the other sideof the membrane. The detected holes may then be repaired by spotelectroplating, such as by using a Hunter Micro-Metallizer Pen availablefrom Hunter Products, Inc., Bridgewater, N.J. In FIG. 23, a patch, orplug, 262 is shown repairing hole 260. Any other suitable method may beused for repairing tiny holes resulting from etching the membrane.

The repairing step of the invented etching process also may be performedusing a photolithographic method. In this case a light-sensitive,electrically insulating mask is applied to one surface of the membrane,and then the membrane is irradiated with light of the appropriatewavelength(s) from the opposite side. Any tiny holes that might bepresent in the membrane will allow the light to pass through themembrane and be absorbed by the light-sensitive mask. Next, the mask iswashed to remove irradiated regions of the mask and thereby reveal thebare metal of the membrane. Because only the irradiated regions of themask are removed, the remaining mask serves as an electrical insulatorover the surface of the membrane. Then, all of the spots where the maskhas been removed are electroplated or electrolessplated at the sametime.

Because the patch, or plug, represents only a minute percentage of thesurface area of the membrane, the patch may be formed from a materialthat is not hydrogen-permeable without the flux through the membranebeing noticeably affected. Of course, a hydrogen-permeable and selectivepatch is preferred. Suitable metals for electroplating to fill or closetiny holes in the palladium-alloy membranes include copper, silver,gold, nickel, palladium, chromium, rhodium, and platinum. Volatilemetals such as zinc, mercury, lead, bismuth and cadmium should beavoided. Furthermore, it is preferable that metal applied by plating berelatively free of phosphorous, carbon, sulfur and nitrogen, since theseheteroatoms could contaminate large areas of the membrane and aregenerally known to reduce the permeability of palladium alloys tohydrogen.

The above-described etched membranes may be used to form membranemodules adapted to be coupled to a source of hydrogen gas, as discussedand/or illustrated herein. The membrane modules include one or morehydrogen-permeable membranes and are adapted to remove impurities from afeed stream of hydrogen gas. It should be understood that the previouslydescribed membrane modules, hydrogen purifiers and fuel processors maybe formed with the invented membranes, but that they may also be formedwith other hydrogen permeable membranes, including unetched membranes.Similarly, the invented etched membranes may be used independent of thepreviously described membrane envelopes, modules, hydrogen purifiers andfuel processors.

INDUSTRIAL APPLICABILITY

The present invention is applicable in any device in which a streamcontaining hydrogen gas is purified to produce a purified hydrogenstream. The invention is also applicable to processes in whichhydrogen-selective membranes are prepared. The invention is alsoapplicable to fuel processing systems in which hydrogen gas is producedfrom a feed stream and subsequently purified, such as for delivery to afuel cell stack or other hydrogen-consuming device.

It is believed that the disclosure set forth above encompasses multipledistinct inventions with independent utility. While each of theseinventions has been disclosed in its preferred form, the specificembodiments thereof as disclosed and illustrated herein are not to beconsidered in a limiting sense as numerous variations are possible. Thesubject matter of the inventions includes all novel and non-obviouscombinations and subcombinations of the various elements, features,functions and/or properties disclosed herein. Similarly, where theclaims recite “a” or “a first” element or the equivalent thereof, suchclaims should be understood to include incorporation of one or more suchelements, neither requiring nor excluding two or more such elements.

It is believed that the following claims particularly point out certaincombinations and subcombinations that are directed to one of thedisclosed inventions and are novel and non-obvious. Inventions embodiedin other combinations and subcombinations of features, functions,elements and/or properties may be claimed through amendment of thepresent claims or presentation of new claims in this or a relatedapplication. Such amended or new claims, whether they are directed to adifferent invention or directed to the same invention, whetherdifferent, broader, narrower or equal in scope to the original claims,are also regarded as included within the subject matter of theinventions of the present disclosure.

We claim:
 1. A fuel processor, comprising: a hydrogen-producing regionadapted to receive a carbon-containing feedstock and water and produce amixed gas stream containing hydrogen gas and other gases therefrom; anda membrane module adapted to receive the mixed gas stream and toseparate the mixed gas stream into a byproduct stream containing atleast a substantial portion of the other gases and a product streamcontaining at least substantially hydrogen gas, the membrane modulecomprising: a hydrogen-selective membrane having a feed side and apermeate side, wherein the product stream is formed from a portion ofthe mixed gas stream that passes through the membrane and the byproductstream is formed from a portion of the mixed gas stream that does notpass through the membrane, wherein the membrane is at leastsubstantially comprised of an alloy comprising palladium and copper andfurther wherein the membrane has an average thickness of approximately25 microns or less, and further wherein the membrane includes at leastone region containing a patch material that is at least substantiallyformed from a material having a different composition than the alloy;and a support adapted to support the membrane, wherein the supportincludes a surface adapted to engage the permeate side of the membrane.2. The fuel processor of claim 1, wherein the patch material is ahydrogen-permeable material.
 3. The fuel processor of claim 1, whereinthe patch material is not a hydrogen-permeable material.
 4. The fuelprocessor of claim 1, wherein the patch material comprises one or moreof the group consisting of copper, silver, gold, nickel, palladium,chromium, rhodium, platinum and mixtures, compounds and alloys thereof.5. The fuel processor of claim 4, wherein the patch material is free ofphosphorous, carbon, silicon, and nitrogen.
 6. The fuel processor ofclaim 5, wherein the patch material is free of zinc, mercury, lead,bismuth and cadmium.
 7. The fuel processor of claim 4, wherein the patchmaterial is free of zinc, mercury, lead, bismuth and cadmium.
 8. Thefuel processor of claim 1, wherein the patch material is free ofphosphorous, carbon, silicon, and nitrogen.
 9. The fuel processor ofclaim 1, wherein the patch material is free of zinc, mercury, lead,bismuth and cadmium.
 10. The fuel processor of claim 1, wherein thepatch material is applied to the membrane after formation of themembrane.
 11. The fuel processor of claim 10, wherein the patch materialis applied to the membrane via a plating process.
 12. The fuel processorof claim 1, wherein the membrane has a thickness of less thanapproximately 20 microns.
 13. The fuel processor of claim 1, wherein themembrane has a thickness of 15 microns or less.
 14. The fuel processorof claim 1, wherein the hydrogen-producing region includes at least onereforming region containing a reforming catalyst bed.
 15. The fuelprocessor of claim 1, wherein the membrane includes at least one etchedregion.
 16. The fuel processor of claim 15, wherein the at least oneetched region includes a central region of the membrane, and furtherwherein the membrane includes an unetched perimeter region of greaterthickness than the central region.
 17. The fuel processor of claim 1,wherein the membrane module includes a pair of the hydrogen-selectivemembranes positioned on opposed sides of the support so that thepermeate surfaces of the membranes generally face each other, with thesupport at least partially defining a harvesting conduit between thepair of membranes and through which the portion of the mixed gas streamthat passes into the conduit through at least one of the pair ofmembranes may be withdrawn from the conduit.
 18. The fuel processor ofclaim 1, wherein the membrane module is adapted to receive the mixed gasstream at a pressure of at least 50 psig and a temperature of at least200° C.
 19. The fuel processor of claim 1, wherein the membrane isadhesively bonded to the support during fabrication of the membranemodule and thereafter subjected to oxidizing conditions after formationof the membrane module.
 20. The fuel processor of claim 1, wherein thesupport is adapted to enable the portion of the mixed gas stream thatpasses through the membrane to flow within the support transverse andparallel to the permeate side of the membrane.
 21. The fuel processor ofclaim 1, wherein the support is at least partially formed from a porousmedium.
 22. The fuel processor of claim 1, wherein the support includesa screen structure having a membrane-contacting screen member.
 23. Thefuel processor of claim 22, wherein the membrane-contacting screenmember is at least partially formed from an expanded metal material. 24.The fuel processor of claim 22, wherein the membrane-contacting screenmember is at least partially formed from mesh.
 25. The fuel processor ofclaim 22, wherein the screen structure includes a plurality of screenmembers.
 26. The fuel processor of claim 25, wherein the plurality ofscreen members are adhesively bonded together during fabrication of themembrane module and thereafter subjected to oxidizing conditions afterformation of the membrane module.
 27. The fuel processor of claim 1,wherein the membrane module further includes end plates between whichthe membrane and the support are supported.
 28. The fuel processor ofclaim 27, wherein the end plates include an inlet port through which atleast a portion of the mixed gas stream is delivered to the permeateside of the membrane, a product outlet port through which the portion ofthe mixed gas stream that passes through the membrane is withdrawn fromthe membrane module, and a byproduct port through which the portion ofthe mixed gas stream that does not pass through the membrane iswithdrawn from the membrane module.
 29. The fuel processor of claim 1,wherein the fuel processor includes a combustion region and furtherwherein the fuel processor includes at least one gas transport conduitadapted to deliver at least a portion of the byproduct stream to thecombustion region.
 30. The fuel processor of claim 1, wherein the fuelprocessor includes a polishing region adapted to receive the portion ofthe mixed gas stream that passes through the membrane and to furtherreduce the concentration of at least a selected component of the othergases therein.
 31. The fuel processor of claim 30, wherein the polishingregion includes at least one methanation catalyst bed.
 32. The fuelprocessor of claim 1, in combination with a fuel cell stack adapted toreceive at least a portion of the product stream and to produce anelectric current therefrom.